Magnetic bearing devices utilizing magnetic force for non-contact support of a rotating piece are widely used in rotating equipment in the Background Art such as turbo molecular pumps that require a rotating piece be rotated at high-speed. Positive features of the magnetic bearing device are that it lowers the rotation resistance of the rotating piece supported by the bearing, generates no particles due to wear, and requires none of the maintenance usually needed due to bearing wear, and no contamination occurs from lubricant fluid in the bearing, etc.
Demands have increased in recent years for magnetic bearing devices that offer a lower cost, more space-saving and higher-speed rotation, etc. The technology of the Background Art has employed sensor-less magnetic bearings that did not require a displacement sensor. Instead of a displacement sensor, sensor-less magnetic bearings has utilized the change in impedance of the electromagnet as one method for detecting displacement of a rotating piece.
Most of the impedance of an electromagnet is made up of the inductance component. The change in this inductance is utilized to detect displacement of the rotating piece. The shape, number of windings, and material of the electromagnet core, as well as the gap between the rotating piece and electromagnet are the main factors in determining the inductance of the electromagnet. The material of the electromagnet core, shape, and the number of windings, are determined in the electromagnet design stage. The change in the inductance of the electromagnet occurs due to a change in the gap between the electromagnet and the rotating piece. In other words, the inductance of the electromagnet changes due to displacement of the rotating piece, and the displacement of that rotating piece can be detected by measuring this change. By feeding back this acquired displacement signal, the rotating piece can be supported in a non-contact levitating state at a specified position.
A non-linear relation is generally established between the magnetic force exerted on the rotating piece and the excitation current of the electromagnet. In the Background Art, a pair of electromagnets are therefore installed facing each other to sandwich the rotating piece, and by then applying a specified direct current bias to each of the opposing electromagnets, a linear relation can be established between the magnetic force exerted on the rotating piece and the excitation current of the electromagnet so that the rotating piece is stably supported in a levitating state as a simple linear system (see Patent Document 1 e.g.).
The method of the Background Art also had the following problems. When there is an actual change in the electromagnet current, then the magnetic characteristics of the electromagnet core change. The inductance of the electromagnet therefore changes even if there is no displacement of the rotating piece. Errors therefore occur when detecting displacement, due to this change in inductance caused by the current of the electromagnet. When applying an external force to the rotating piece via the electromagnets in general, the rotating piece displacement is large for the force at low frequencies, and the rotating piece displacement is small for the force at high frequencies. The change in inductance at low frequencies therefore causes a larger change in rotating piece displacement than from changes due to the electromagnet current, and there is little effect from displacement detection errors induced by the electromagnet current. However, the change in inductance at high frequencies renders the opposite effect in case that changes due to the electromagnet current are larger, and the effect to displacement detection errors is large. Therefore, control of the magnetic bearing tended to be unstable in the high frequency range.
As one countermeasure to this problem of unstable magnetic bearing control in the high frequency range, the Background Art as shown in the Patent Document 1 employed a method to detect the electromagnet current and eliminate the differential in predicted displacement detection error from the displacement detection signal that was detected via the change in impedance of the electromagnets (Patent document 1 e.g.).
[Patent Document 1] Laid Open Patent Application 2004-132537